The present invention relates to a multiplex transmission method for motion picture signals and an apparatus therefor and, more particularly, to a multiplex transmission method for motion picture signals, in which motion picture signals from a plurality of channels are encoded by using a band compression technique, and multiplex transmission of the encoded data is performed by using a band-limited line, and an apparatus therefor.
As a conventional multiplex transmission method for motion picture signals, a method is disclosed in "Statistical Performance Analysis of an Interframe Encoder for Broadcast Television Signals", IEEE TRANSACTIONS ON COMMUNICATIONS, COM-29, No. 12, 1981, pp. 1,868-1,876.
This method is based on the fact that the statistical characteristics of input motion picture signals between a plurality of channels are generally independent of each other with respect to multiplex transmission of a plurality of encoded channel motion picture signals, and there is no correlation between the amounts of generated information in encoding the signals. That is, there is a very low probability that the amounts of encoded information output from the respective encoders are maximized at the same time. In addition, no substantial deterioration in image quality occurs in a channel with a small amount of generated information even if the transmission rate is low. Therefore, an excess portion can be subtracted from the transmission rate allocated to the channel, and the corresponding portion can be allocated to a channel with a large amount of generated information, thereby achieving an improvement in image quality. Obviously, the total transmission rate remains the same.
FIG. 12 shows a multiplex transmission system to which such a conventional multiplex transmission method for a plurality of motion picture signals is applied.
Referring to FIG. 12, different motion picture signals from image channels CH1 to CHn are generally supplied to encoders A1 to An corresponding to the image channels, respectively. The encoders A1 to An then encode the signals. Each of the encoders A1 to An supplies encoded data 91, including video and audio information and information required for decoding an encoded image at the receiving end, to a variable rate multiplexer 94 together with information 90 indicating the amount of generated information. The variable rate multiplexer 94 adaptively determines allocation of transmission rates to the respective channels at predetermined intervals, e.g., 125 .mu.sec, by referring to the information 90 indicating the amount of generated information, which is supplied from each of the encoders A1 to An (when the amount of generated information in a given image channel is large, part of the information is allocated to another transmission channel with a small amount of generated information, i.e., determination of adaptive multiplexing is performed). The variable rate multiplexer 94 then supplies read clocks 93 corresponding to transmission rates allocated to the respective channels to the encoders A1 to An, respectively.
The encoders A1 to An read the encoded data 91 in accordance with the read clocks supplied from the variable rate multiplexer 94, and supply the read data to the variable rate multiplexer 94. The variable rate multiplexer 94 adds pieces of image channel identification information to the encoded data 91 supplied from the encoders A1 to An and multiplexes them. The variable rate multiplexer 94 then outputs the resultant data to a transmission line L having a large-capacity fixed transmission rate.
Each of the encoders A1 to An includes a buffer memory for leveling the amounts of the encoded data 91, which undergoes a great variation instantly, at an output timing. With this arrangement, the encoders A1 to An can properly respond to a read operation of the variable rate multiplexer 94 at variable rates.
At the receiving end, the multiplexed encoded data of the plurality of channels, which are supplied through the transmission line L having the large-capacity fixed transmission rate, are received, and each identification information is detected. A variable rate separator 95 then separates the encoded data 91 into encoded data 96 for the respective image channels. The separated encoded data 96 of the respective channels are supplied to decoders B1 to Bn, respectively, together with write clocks 97. The encoded data 96 are then decoded by the decoders B1 to Bn for the respective channels, thereby obtaining reproduced images.
By performing such an adaptive multiplex operation, a substantially high transmission rate can be allocated to a motion picture signal of a channel exhibiting the maximum amount of generated information. Therefore, a great reduction in encoding distortion can be achieved. In addition, the encoding distortion of a motion picture signal exhibiting the minimum amount of generated information does not increase, and hence an improvement in overall image quality can be realized.
In order to efficiently use a transmission line and transmit motion picture signals by performing variable rate multiplexing with respect to a plurality of channels in accordance with the amounts of generated information in the above-described conventional multiplex transmission apparatus for motion picture signals, the transmission line needs to have a high transmission rate of several tens Mb/s corresponding to a plurality of channels, or an ultra-high transmission rate of several hundreds Mb/s as the number of multiplex channels increases. However, in a satellite circuit or even in a terrestrial circuit, if the band is greatly limited, such an ultra-high rate transmission line cannot be ensured.
Furthermore, according to the conventional multiplex method, since separation of all channels is required at the receiving end, decoders equal in number to the multiplex channels at the transmitting end must be prepared and used, and decoding cannot be performed with respect to only a desired channel. These channels, however, are not necessarily used all the time. Furthermore, in many instances, the receiving end need not receive or decode signals from all the channels.
For this arrangement, the size of an apparatus at the receiving end inevitably increases.